assert_response_success() in phpbb_functional_test_case checks that "Warning:" should not exist but it may not neccessarily be a PHP Warning. e.g http://localhost/phpBB/adm/index.php?sid=78ff693cc778f14ae090aafdc7db6e1b&i=acp_board&mode=security
"Warning: Only enable this.. " is a string being used inside phpBB